Growth in the use of masks for photolithography, especially immersion photolithography, where structure sizes can be as small as 45 nm or less, has caused mask structures to become increasingly complex. Special design programs are used to produce the mask design, i.e., essentially the locations of the structures on the mask, convert it to a mask layout that incorporates lithographic requirements (e.g., the photoresist used, the light source used, the imaging scale), and store it in a file. The mask layout is the basis for the production of the mask. Due to the complexity of mask structures, the production costs of photolithography masks, especially those suitable for immersion lithography, are quite high. To prevent driving these costs still higher, defects or errors in the production process are repaired whenever possible.
In some examples, to find defects on masks, after the requisite production steps, the masks are checked for defects by using inspection systems. The inspection systems generate position files in which the positions where defects exist are stored. The position files may include a classification of the defects found at a given position according to predefined categories. The position files, in turn, serve as inputs for “mask repair systems,” such as, for example, MeRiT® system, available from Carl Zeiss SMS GmbH. From the inspection system, the mask is routed to the mask repair system. In the mask repair system, the defect positions stored in the position file are navigated to, one by one, and an effort is made to repair the defect, for example by depositing or removing material.
After all the defects have undergone repairs, the result of the repair is verified for each defect. The verification should take place under conditions that reflect the imaging conditions in the photolithography scanner as closely as possible. In particular, this calls for optical qualification under the same lithographically relevant conditions as in the photolithography scanner. For example, the wavelength, the numerical aperture and the illumination setting (such as dipole or quadrupole illumination) of the imaging system used for the verification are matched to those of the photolithography scanner. The verification can, for example, be performed using an emulation imaging system that emulates a photolithography scanner, in which an image of the mask, instead of being displayed in reduced form on the photoresist on a wafer (as in a photolithography scanner), is displayed in magnified form on a spatially resolving detector, for example a charge coupled device (CCD) camera. One such simulation imaging system is AIMS® system, available from Carl Zeiss SMS GmbH. This machine navigates to the repaired position and takes a corresponding image of the repaired site, known as an aerial image (equivalent to an image taken in a photoresist layer above the wafer), on the mask at the repaired position.
In some examples, whether the repair was successful and the mask can thus be verified is determined by manual comparison with a structure at another point on the mask that is identical but has no defect, and thus did not have to be repaired. Tolerance criteria are defined for these target parameters, and consideration may also be given to, for example, the behavior and/or the values of the target parameters at the identical, unrepaired site. Based on the aerial image, compliance with the tolerance criteria is then checked using evaluation algorithms. If the tolerance criteria are met, then any deviations from the ideal value are within tolerance, the repair at the position under examination is verified, and the position can be marked accordingly in the position file or, alternatively, deleted from the position file. If the tolerance criteria are not met, then the position can also be marked accordingly in the position file so it can undergo another repair.
The comparison and the verification of repairs can be done manually: a user looks for an identical, unrepaired site near the repaired site and analyzes and compares the two images. This is time-consuming, especially in cases where a similar structure can be found only outside the image area of the emulation imaging system.
Another prerequisite for this type of verification is that the mask needs to have similar structures that can be used to make the comparison. In the case of masks for logic circuits, however, this condition is not always met. The masks for logic circuits tend to feature a large number of structures, each of which occurs only once. In this case it is difficult to achieve relatively precise verification of repairs, and an approximate verification by reference to similar structures is performed.